A frequency converter is used to convert a first input frequency to a higher or a lower second output frequency. For example, in certain transmitters, a baseband signal such as a television signal or a radio signal, or an Intermediate Frequency (IF), have to be upconverted to, for example, a Very High Frequency (VHF) signal or an Ultra High Frequency (UHF) signal within a certain channel or allotted frequency band. Similarly, at a receiver receiving the VHF or UHF signal, the VHF or UHF signal has to be downconverted to a baseband or an IF signal usable by the receiver.
In a known upconversion technique, an incoming baseband or IF signal is multiplied in a single Mixer with a signal produced by a Local Oscillator (LO) (e.g., a carrier signal) to directly produce an upconverted output signal that occupies a desired VHF or UHF channel or frequency band. A filter is used to pass one sideband of the mixing process that falls within the desired VHF or UHF channel or frequency band, and to block any undesired sidebands or a LO signal at the output of the Mixer from being transmitted. The problem with the single mixer upconverter is that generally at least one of the upper and lower sidebands and the LO signal fall within the higher VHF or UHF channel. As a result, a special bandpass filter is needed to just pass, for example, the upper sideband signal and reject the lower sideband and LO signal. Alternatively, a well-known tracking filter can be used that only tracks the desired output signal. Both of the special bandpass and tracking filters are difficult and expensive to implement. This limits the choice of options of the filters and other devices that can be used.
Upconverters which are programmable such that they can be utilized to generate signals on any selected channel within the VHF or UHF frequency band are known as "agile". Most "agile" conventional upconverters use a two stage (or double) conversion technique. The first stage of the conversion brings the incoming signal to a high frequency IF, and the second stage of the conversion brings the high frequency IF to the desired channel frequency. Each stage includes a mixer which receives an oscillator signal. The oscillator can comprise a programmable phase locked loop which has its output frequency regulated in accordance with a comparison between a sampling of the oscillator output signal (which is changeable) and a reference signal.
U.S. Pat. No. 5,038,404 (D. Marz), issued on Aug. 6, 1991, discloses an exemplary double upconversion technique that provides agility. More particularly, this upconverter comprises first and second mixer stages in cascade, where each mixer stage includes a separate programmable phase locked loop with an oscillator (LO) located therein which is controlled by a phase comparator or detector. The first mixer stage receives an IF signal [e.g., a 45.75 MHz signal for a National Television Systems Committee (NTSC) system], and upconverts the IF signal to a high IF signal (e.g., 720 MHz) which is above a desired output channel frequency range. The second mixer stage then downconverts the high IF signal to the frequency range of the desired UHF or VHF output channel. In this double upconversion arrangement, the first Local Oscillator associated with the first mixer stage has a relatively narrow or small tuning range (e.g., 3 MHz), while the second Local Oscillator associated with the second mixer stage has a very wide or large tuning range (e.g., 500 to 1000 MHz). As described in U.S. Pat. No. 5,038,404, an oscillator with a large tuning range suffers from a poor phase noise. It is also known that it is possible to design an oscillator with a narrow tuning range that provides excellent phase noise.
In the conventional upconverter described in U.S. Pat. No. 5,038,404 (or in a comparable downconverter), the output frequency of the upconverter can be changed by changing the first stage LO with the narrow tuning rage, the second stage LO with the wide tuning range, or both. When the first stage LO is changed while the second stage LO is kept unchanged, a small frequency change or step is achieved in the upconverted output signal. When the second stage LO is changed while the first stage LO is kept unchanged, a large frequency change or step is achieved in the upconverted output signal. Both LOs must be changed if an output frequency change is required which is larger than the large step change provided by the second stage LO but smaller than two times the smaller step change provided by the second stage LO to provide additive step changes. The problem in prior art upconverters having such first stage narrow tuning range oscillator and second stage wide tuning range oscillator, is that for making very small frequency step changes (in the order of KHz as required for 2.5 KHz frequency step Cable Television applications), the first narrow tuning range LO phase locked loop is very slow. Still further, if the first stage LO phase locked loop is disturbed (e.g., the upconverter is accidently bumped), the first stage LO will be slow in reacquiring lock due to the narrow band loop. In certain situations as, for example, the digital transmission of television signals, this is not acceptable because it results in a loss of a television picture, and an interruption of more than a second for the receiver at a customer's location to reacquire the picture.
It is desirable to provide an upconverter or a downconverter which allows for both small and large step converter output signal changes and achieves proper phase noise criteria while allowing for very fast reacquisition of the phase locked loops if any one of the first and second stage LO phase locked loops are disturbed.